The present invention relates to an electrophotographic plate for lithographic printing and a lithographic printing plate made therefrom.
Hitherto, as printing plates for lithographic printing, there have been known those which use light sensitive resins and silver halide light sensitive materials. The former are superior in resolution and printing endurance, but low in sensitivity and the latter are high in sensitivity and resolution, but expensive and inferior in printing endurance.
Lithographic printing plates of electrophotographic type which use photoconductive materials are relatively high in sensitivity and can provide inexpensive, high endurance plates and recently have been studied intensively. Printing plates of this type are made by forming toner images on a photoconductive layer provided on a support by an electrophotographic method and then rendering the non-image area hydrophilic or dissolving out the non-image area. For example, in the case of printing plates comprising a support and, provided thereon, a photoconductive layer comprising a binder, zinc oxide as a photoconductive material and a sensitizer, the non-image area is rendered hydrophilic by, for example, potassium ferrocyanide to produce printing plates. However, hydrophilicity of the non-image area is inferior and hence the plates are easily stained and, further, are low in printing endurance. There is another printing plate having a hydrophilic aluminum plate as a support and the non-image area is dissolved out, but the photoconductive layer contains zinc oxide in a large amount and so dissolving out of the non-image area is difficult and the surface of the hydrophilic aluminum plate cannot be sufficiently utilized. An example of using an organic photoconductor as a photoconductive material is disclosed in Japanese Patent Unexamined Publication (Kokai) No. 107246/81, a photoconductive layer comprising a binder soluble in aqueous alkali or alcohol solutions, a large amount of an oxadiazole organic photoconductor and a small amount of a sensitizing dye is coated on an aluminum plate. In this case, however, since oxadiazole photoconductor is contained in a large amount, dissolution of the photoconductive layer is not necessarily satisfactory, the oxadiazole organic photoconductor is often precipitated and sensitivity is low. In order to improve these defects, there have been proposed printing plates the photoconductive layer of which comprises two layers, a charge generation layer and a charge transport layer to improve sensitivity by reducing the content of the organic photoconductor, but these printing plates have two-layer construction and hence the cost for production increases.
Furthermore, there are proposed printing plates the photoconductive layer of which comprises a dispersion of various organic photoconductive pigments as the organic photoconductor in a binder soluble in aqueous alkali or alcohol solutions.
For example, in the case of a printing plate comprising an aluminum plate on which is provided a photoconductive layer comprising a dispersion of phthalocyanine pigment in phenolic resin, a sharp increase in sensitivity cannot be expected and hence photoconductors such as said oxadiazole type, pyrazoline type and hydrazone type photoconductors are added, resulting in a reduction of charge potential, precipitation of the photoconductor and deterioration in dissolution. Further, Japanese Patent Unexamined Publication (Kokai) No. "146135/81" uses a printing plate having a light sensitive layer containing condensed polycyclic quinone pigment and an oxadiazole derivative as the organic photoconductor, but this plate is still insufficient in sensitivity and besides, there is the possibility of the precipitation of oxadiazole.